Cf252 Neutron and Gamma (no shielding) -- PE Counts

These are a comparison between Cf252 Neutrons and Gammas, placed 20.7 cm above the plastic scintillator slab, with no shielding.

Red = 1/2 inch Pb with poly bucket

Blue= 1 inch Pb with poly bucket

The stats box applies to the 1 inch Pb results

Figure 1: 3 PE Trigger, SiPM A	Figure 2: 3 PE Trigger, SiPM B
Figure 3: 3 PE Trigger, SiPM C	Figure 4: 3 PE Trigger, Sum of SiPMs A,B,C
Figure 5: 3 PE Trigger, Sum of SiPMs B,C

The above graphs seem to indicate that 1/2 inch Pb in the shielding bucket is just not enough to attenuate the Cf252 gammas. We must use a 1“ layer instead.

This is supported by the effects on the gamma spectra:

Blue = Unshielded Red = Shielded primaries Green = Secondaries generated in shielding

Figure 6a: Gamma Energy Spectrum, 1/2” Pb	Figure 6b: Number of Gammas Per Event Which Reached Scintillator (“Gamma Count Spectrum”)

Figure 7: Gamma Energy Spectrum, 1“ Pb	Figure 8: Gamma Energy Spectrum, 1” Pb, zoom	Figure 8b: Gamma Count Spectrum, 1“ Pb

Also, the increased shielding doesn't significantly reduce the neutron flux to the surface of the scintillator, though it does reduce the energies somewhat:

Figure 9: Neutron Energy Spectrum, 1/2” Pb	Figure 10: Neutron Energy Spectrum, 1/2“ Pb, zoom	Figure 10b: Neutron Count Spectrum, 1/2” Pb

Figure 11: Neutron Energy Spectrum, 1“ Pb	Figure 12: Neutron Energy Spectrum, 1” Pb, zoom	Figure 12b: Neutron Count Spectrum, 1“ Pb

Figure 13: Neutron Energy Spectrum, Thermal Neutron Zoom (~eV) 1/2” Pb	Figure 14: Neutron Energy Spectrum, Thermal Neutron Zoom (~eV), 1“ Pb

These neutrons don't generate very many gammas as they pass through the bucket. Most of these gammas are generated due to neutron capture on H in the poly layer of the bucket. The 1” Pb shield seems to generate more of these gammas (especially gammas due to other processes than neutron capture on H). Also, from the comparison between figures 13 and 14, we can see that the rate of thermal neutron generation between the 1/2“ and 1” Pb-insert buckets is about the same, so using 1“ won't lead to a significantly increased thermal neutron flux onto the scintillator slab.

Figure 11: Gamma Energy Spectrum, Neutron Primaries, 1/2” Pb	Figure 12: Gamma Energy Spectrum, Neutron Primaries, 1/2“ Pb
Figure 11: Gamma Energy Spectrum, Neutron Primaries, 1” Pb	Figure 12: Gamma Energy Spectrum, Neutron Primaries, 1“ Pb